Over the years, my washable furnace air filter has saved me hundreds of dollars. The coils for the a/c and heat exchanger have remained clean, so it must be working, at least pretty well. It's getting harder to give it a really good cleaning and was considering replacement. Anyone have any recommendations? Thoughts, pro/con?

Don't know, but I'm curious as to what you learn. Have you ever tried to measure DP relative to other standard throwaway filters?

When my new system was installed I soon dialed back to the lowest restriction pleated (and electrostatically charged) filter I could find on local shelves. The installer put in a simple flat screen "filter" that had next nothing in the way of filtration ability...and little DP. I then put in a generic pleated filter of modest claims...and immediately noted a very high filter DP--high enough that I created a simple tubing manometer to measure it, then measured the blower power draw and said, "Hell, no!" Larger boots (matching the original furnace filter size rather than the new one) in standard filter sizes are no longer sold, so I decided to reduce the media DP. I picked up a low rated "Filtrete 600" pleated filter. It provided lower DP, but still seems to filter properly.

I've used low restriction filters (K&N's with low restriction velocity stacks) in cars I modified for greater horsepower and torque, never had any problems with them. Washed them and reoiled about twice a year. Engines never suffered over 100K of nearly daily redline revs, and I measured substantial performance gains from the filters alone.

Depending upon the type of media on the inside of your air filter, it may or may not be a life time filter.
Polyester will clog up over time and can not be cleaned. Foam will deteriorate. You should use a mild degreaser and rinse with water. You may need to use a hose that has strong pressure. The spray nozzle in the sink may not have enough pressure to push the trapped particulate out.
Hope this helps!
airclean.com

I looked into this recently, mostly by following blogs on another DIY site that has a lot of HVAC professionals, and the consensus there was not to use a "permanent" filter. Most of the pro HVAC guys could give a number of anecdotes of failed washable filters, and in particular they observed very dirty A-coils with them.

One of the pros said to basically look for a MERV 8 paper filter and call it good. That's what I've been doing. Have been burned by the high efficiency Filtrete filters, which are so efficient they clog up a little too early for my tastes - nothing like the "90 day" claims they make.